1. Field of the Invention
The present invention relates to an image reading apparatus having a color image pick-up device.
The present application is based on Japanese Patent Application Nos. Hei. 11-121607 and 2000-90792, which are incorporated herein by reference.
2. Description of the Related Art
Some of the heretofore known image reading apparatus for reading an image on an original by moving a carriage in parallel to the surface of the original, the carriage being loaded with a color image pick-up device having line sensors such as three of CCDs including a number of pick-up elements arranged linearly for respectively reading three primary colors.
In the case of an flat-bed type image reading apparatus, for example, an original glass plate formed with a transparent plate such as glass for placing an original thereon is provided on the surface of a box-like casing. A carriage moved by a driving unit in parallel to the original glass plate is provided in the casing. The carriage is loaded with a light source and a color image pick-up device. The light emitted from the light source is reflected from the surface of the original on the original glass plate and concentrated by a condenser lens on the color image pick-up device.
In the image reading apparatus, increasing the number of pick-up elements constituting the CCD is needed to improve read resolution in the main scanning direction as the arrangement direction of the pick-up elements in the CCD. However, the CCD tends to increase in size when the number of pick-up elements is increased while the size of the individual elements remains equal and there is a problem arising from an increase in costs as the load of designing an optical system increases. On the other hand, decreasing the size of each element also develops a problem arising from limitations on production.
As disclosed in Unexamined Japanese Patent Publication No. Sho. 58-19081, there is known a CCD image sensor having a first optical detector train and a second optical detector train that is shifted from the first optical detector by about half the width of the individual optical detectors. In this CCD image sensor, the first optical detector train is set adjacent to the second optical detector train in the sub-scanning direction.
With the CCD image sensor having the plurality of optical detector trains, read resolution in the main scanning direction can be doubled in comparison with a CCD image sensor having a single optical detector train because the same line on the original is to be read by the first and second optical detector trains.
The output of the pick-up element in the CCD varies with the quantity of incident light and the charge accumulation time required. As the product of the quantity of light and the charge accumulation time increases, the output proportionally increases up to a predetermined level. However, the pick-up element has such characteristics that the output is saturated when the output reaches the predetermined level and never increased by increasing the quantity of incident light and prolonging the charge accumulation time. Since the output of the pick-up element contains an offset level due to a dark current, the dynamic range of the pick-up element is from the offset level up to the predetermined level.
In such an apparatus as this, it has been practiced to lower the offset level as a noise level when the original is read in high quality by reducing the dark current by cooling the CCD image sensor using a Peltier-device or the like. Thus, a high-quality image can be read because the dynamic range obtained from the output of the CCD image sensor is expandable and the S/N ratio is improvable.
Notwithstanding, an expensive part such as the Peltier-device needs using when the high-quality image is read through the method stated above and this also develops the problem of making the apparatus complicated, thus increasing the cost.